This application claims the benefit of Korean Application No. 2000-76492 filed Dec. 14, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a solid immersion mirror (SIM) type objective lens and an optical pickup device adopting the same, and particularly, to a modified SIM type objective lens for far field recording/reproducing, and an optical pickup device adopting the same.
2. Description of the Related Art
In general, information recording/reproducing density increases as a size of a light spot formed on an optical disc by an optical pickup device is decreased. The size of a light spot decreases as a wavelength of a light beam in use becomes shorter and an NA of an objective lens which focuses the light spot becomes greater. A relationship of the size of a light spot to wavelength and NA is shown in equation (1).
Size of light spotxe2x88x9dxcex/NAxe2x80x83xe2x80x83(1) 
Thus, an optical pickup device for high density should adopt a light source emitting a light beam having a shorter wavelength and an objective lens having a high NA. To realize a stable system, the objective lens should have a working distance (a distance from a light exit surface of an objective lens to a light input surface of an optical disc) which is large. An optical pickup device for recoding/reproducing information on/from an optical disc of a next generation DVD family, or a so-called HD-DVD (high definition digital versatile disk) family, may adopt, for example, a light source for emitting a light beam having a wavelength of 405 nm and an objective lens having an NA of 0.85 and a large working distance.
However, due to a limit in manufacture, it is difficult to manufacture an objective lens formed of a single lens having an NA of 0.7 or more and satisfying an allowance condition of optical aberration. Thus, to realize an NA of 0.7 or more and satisfy an allowance condition of optical aberration, an objective lens 10 formed of two lenses as shown in FIG. 1 has been suggested.
Referring to FIG. 1, a conventional objective lens 10 includes a first condensing lens 11 for condensing incident light and a second condensing lens 13 arranged between the first condensing lens 11 and an optical disc 1 for increasing an NA of the objective lens 10. In the objective lens 10, for example, where a 0.6 NA is secured by the first condensing lens 11, the NA may be increased by the second condensing lens 13. For the objective lens 10 to have a 0.85 NA, a light input surface of each of the first and/or second condensing lenses 11 and 13, facing a light source (not shown), is formed to have a large curvature, or at least one of the first and second condensing lenses 11 and 13 is formed of a material exhibiting a high refractive index, to produce a sharp refraction of light.
Thus, the objective lens 10 as shown in FIG. 1 is sensitive to decenter, being off an optical axis, and coma is greatly generated according to an amount of the decenter. Also, the objective lens 10 is difficult to manufacture because processing a lens surface having a large curvature is difficult.
Also, the working distance WD1 of the objective lens 10 is short, for example, about 0.15 mm due to a sharp refraction of light. It is difficult to design the objective lens 10 to have a working distance of 0.15 mm or more. For reference, the working distance of an objective lens in an optical pickup device for DVD is about 1.8 mm.
Since the objective lens 10 realizes a high NA by the structure of two lenses, where the first and second condensing lenses 11 and 13 are inclined to each other, it is impossible to maintain a small optical aberration. Thus, allowance of distance and inclination between the first and second condensing lenses 11 and 13 is very strictly obeyed.
Referring to FIG. 2, a conventional solid immersion mirror 20 includes a first transmission surface 21 for diverging and transmitting incident light, a second transmission surface 23 disposed to face the first transmission surface 21, a first reflection surface 25 formed around the second transmission surface 23, for reflecting incident light passing through the first transmission surface 21 and a second reflection surface 27, formed around the first transmission surface 21, for reflecting incident light reflected by the first reflection surface 25 to proceed toward the second transmission surface 23.
The solid immersion mirror 20 as described above may realize an NA of 0.7 or more with a single lens structure. In the solid immersion mirror 20, since a blocking area, indicated by a hatched area 29 in FIG. 2, exists where the light input to the first transmission surface 21 is relatively near the optical axis, some of the input light is not focused on a recording surface of the optical disc 1 and is lost. Here, the light lost by not being focused on the recording surface of the optical disc 1 is light directly input to the second transmission surface 23 from the first transmission surface 21, and light which is lost at a boundary between the first transmission surface 21 and the second reflection surface 27 among the light from the fist transmission surface 21, reflected by the first reflection surface 25, and proceeding toward the second reflection surface 27. In FIG. 2, to show the blocking area, only a proceeding path of the light input to the first transmission surface 21 is shown. Light reflected by the recording surface of the optical disc 1 and input to the second transmission surface 23 proceeds in the reverse order along the light proceeding path shown in FIG. 2.
The solid immersion mirror 20 can realize a high NA of 0.7 or more with a single lens. Since the solid immersion mirror 20 has a structure in which light is condensed after reflected from the two reflection surfaces 25 and 27, curvature may be small so that the solid immersion mirror 20 may be insensitive to the decenter, exhibit relatively superior chromatism quality, and be manufactured easily.
However, since the blocking area exists due to the structure of the solid immersion mirror 20, as shown in FIG. 2, all of the incident light is not used, and the efficiency of light is reduced. About {fraction (1/3)} of the incident light is blocked in the conventional solid immersion mirror 20.
Also, since the quantity of the blocked light depends on the size of the first transmission surface 21, the diameter of the first transmission surface 21 is made less than xc2xc of an overall effective diameter of the solid immersion mirror 20 to minimize reducing the light efficiency. To maximize efficiency in an optical pickup device employing the solid immersion mirror 20 as an objective lens, an incident light beam having a diameter slightly greater than that of the first transmission surface 21 is input to the first transmission surface 21. Accordingly, the quantity of light input to the first transmission surface 21 is greatly affected by movement of the solid immersion mirror 20 for tracking in a radial direction perpendicular to the optical axis.
FIG. 3 shows a light beam intensity profile of light focused by the solid immersion mirror 20. As can be seen from FIG. 3, since a side lobe S1 which amounts to 5-6% of a peak value of the light intensity is relatively great, where the solid immersion mirror 20 is adopted as an objective lens of an optical pickup device, a great amount of jitter is generated during reproduction of information recorded on the optical disc 1 and a cross erasure problem of removing signals recorded on adjacent tracks may occur during recording. The conventional solid immersion mirror 20 used to generate the light intensity profile shown in FIG. 3 has an NA of 0.85, an overall effective diameter of 4.5 mm, and the first transmission surface 21 has a diameter of 1.0 mm. Thus, where about 33% of the quantity of light input to the first transmission surface 21 is blocked. FIG. 3 shows a light intensity profile of a beam having a 400 mm wavelength and focused by the conventional solid immersion mirror 20. A side lobe is also produced in light focused by the objective lens 10 as shown in FIG. 1. However, the side lobe produced in light focused by the objective lens 10 is about 2-3% or less of a peak value of the light intensity so that it does not cause a serious increase of jitter in a reproduction signal and cross erasure.
Further, since the conventional solid immersion mirror 20 having the structure shown in FIG. 2 has a short working distance WD2, the SIM 20 is usable as an objective lens for high density light condensation in a near field recording/reproducing needing a working distance of several tens to hundreds nano meters, but is not useable as an objective lens for far field recording/reproducing.
To solve the above-described and other problems, it is a first object of the present invention to provide a solid immersion mirror type objective lens formed of a single lens and having a high NA of 0.7 or more.
It is a second object of the invention to provide an optical pickup device which adopts the solid immersion mirror type lens formed of a single lens and having a high NA of 0.7 or more. The solid immersion mirror type objective lens of the present invention has a superior efficiency of light and a drastically reduced side lobe component compared with a conventional solid immersion mirror. Thus, where the solid immersion mirror type objective lens of the present invention is adopted in an optical pickup device, excess jitter is not generated during reproducing and cross erasure does not occur during recording. Also, since a relatively great working distance (compared with a conventional solid immersion mirror) is realizable, the solid immersion mirror type objective lens of the present invention is applicable to far field recording/reproducing.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the first and other objects of the invention, there is provided an objective lens comprising a first transmission surface formed of a first curved surface which condenses incident light and a second curved surface formed around the first curved surface which diverges incident light, a second transmission surface disposed to face the first transmission surface and which transmits incident light, a first reflection surface formed around the second transmission surface which reflects incident light, and a second reflection surface formed around the first transmission surface which reflects incident light.
To achieve the second and other objects of the invention, there is provided an optical pickup device comprising a light source which generates and emits a laser beam, an optical path changer which changes a proceeding path of incident light, an objective lens arranged on an optical path between the optical path changer and a recording medium which condenses incident light emitted from the light source to form a light spot on the recording medium, and a photodetector which detects incident light reflected by the recording medium and passing through the objective lens and the optical path changer, wherein the objective lens comprises a first transmission surface comprising a first curved surface and which condenses incident light and a second curved surface formed around the first curved surface which diverges incident light, a second transmission surface disposed to face the first transmission surface which transmits incident light, a first reflection surface formed around the second transmission surface which reflects incident light, and a second reflection surface formed around the first transmission surface which reflects incident light.